[PATCH 3/4] xfs/420: fix occasional test failures due to pagecache readahead

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

Every now and then, this test fails with this golden output:

--- xfs/420.out
+++ xfs/420.out.bad
@@ -29,7 +29,7 @@
 Whence Result
 DATA   0
 HOLE   131072
-DATA   196608
+DATA   192512
 HOLE   262144
 Compare files
 c2803804acc9936eef8aab42c119bfac  SCRATCH_MNT/test-420/file1

Curiously, the file checksums always match, and it's not *forbidden* for
the page cache to have a page backing an unwritten extent that hasn't
been written.

The condition that this test cares about is that block 3 (192k-256k) are
reported by SEEK_DATA as data even if the data fork has a hole and the
COW fork has an unwritten extent.  Matthew Wilcox thinks this is a side
effect of readahead.

To fix this occasional false failure, call SEEK_DATA and SEEK_HOLE only
on the offsets that we care about.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 tests/xfs/420 |   33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)


diff --git a/tests/xfs/420 b/tests/xfs/420
index 12b17588..d38772c9 100755
--- a/tests/xfs/420
+++ b/tests/xfs/420
@@ -50,6 +50,24 @@ _scratch_mount >> $seqres.full 2>&1
 testdir=$SCRATCH_MNT/test-$seq
 mkdir $testdir
 
+# pagecache readahead can sometimes cause extra pages to be inserted into the
+# file mapping where we have an unwritten extent in the COW fork.  Call lseek
+# on each $blksz offset that interests us (as opposed to the whole file) so
+# that these extra pages are not disclosed.
+#
+# The important thing we're testing is that SEEK_DATA reports block 3 as data
+# when the COW fork has an unwritten mapping and the data fork has a hole.
+exercise_lseek() {
+	echo "Seek holes and data in file1"
+	$XFS_IO_PROG -c "seek -d 0" $testdir/file1
+	$XFS_IO_PROG -c "seek -h $((2 * blksz))" $testdir/file1 | sed -e '/Whence/d'
+	echo "Seek holes and data in file2"
+	$XFS_IO_PROG -c "seek -d 0" $testdir/file2
+	$XFS_IO_PROG -c "seek -h $((2 * blksz))" $testdir/file2 | sed -e '/Whence/d'
+	$XFS_IO_PROG -c "seek -d $((3 * blksz))" $testdir/file2 | sed -e '/Whence/d'
+	$XFS_IO_PROG -c "seek -h $((4 * blksz))" $testdir/file2 | sed -e '/Whence/d'
+}
+
 blksz=65536
 nr=8
 filesize=$((blksz * nr))
@@ -83,10 +101,7 @@ $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file1 >> $seqres.full 2>&1
 $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file2 >> $seqres.full 2>&1
 $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file3 >> $seqres.full 2>&1
 
-echo "Seek holes and data in file1"
-$XFS_IO_PROG -c "seek -a -r 0" $testdir/file1
-echo "Seek holes and data in file2"
-$XFS_IO_PROG -c "seek -a -r 0" $testdir/file2
+exercise_lseek
 
 echo "Compare files"
 md5sum $testdir/file1 | _filter_scratch
@@ -102,10 +117,7 @@ $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file1 >> $seqres.full 2>&1
 $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file2 >> $seqres.full 2>&1
 $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file3 >> $seqres.full 2>&1
 
-echo "Seek holes and data in file1"
-$XFS_IO_PROG -c "seek -a -r 0" $testdir/file1
-echo "Seek holes and data in file2"
-$XFS_IO_PROG -c "seek -a -r 0" $testdir/file2
+exercise_lseek
 
 echo "Compare files"
 md5sum $testdir/file1 | _filter_scratch
@@ -121,10 +133,7 @@ $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file1 >> $seqres.full 2>&1
 $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file2 >> $seqres.full 2>&1
 $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file3 >> $seqres.full 2>&1
 
-echo "Seek holes and data in file1"
-$XFS_IO_PROG -c "seek -a -r 0" $testdir/file1
-echo "Seek holes and data in file2"
-$XFS_IO_PROG -c "seek -a -r 0" $testdir/file2
+exercise_lseek
 
 echo "Compare files"
 md5sum $testdir/file1 | _filter_scratch